2.  Terms, definitions and abbreviated terms

No terms and definitions are listed in this document.

1.  Introduction

This OGC Testbed 18 Engineering Report presents the result of research performed as a component of the 3D+ Data Standards and Streaming thread. A review of existing standards pertaining to the efficient streaming and visualization of spatiotemporal 3D and 4D data was performed. Use cases in space, including objects referenced relative to a spherical body other than Earth, in orbit, or in free flight within the Solar System, the Milky Way, or outside our galaxy were also considered.

The relevant standards and candidate standards identified include the following.

• Multiple OGC APIs

  • OGC API — 3D GeoVolumes for streaming 3D content referenced to a spherical body

  • OGC API — Tiles for streaming arbitrary content referenced to a spherical body

  • OGC API — Moving Features for streaming the changing positions and properties of objects over time

  • OGC API — Connected Systems for streaming the positions of many objects or of objects moving rapidly

• The OGC 2D Tile Matrix Set and Tileset Metadata as the foundation for OGC API — Tiles as well as the fixed tiling of 3D space requirements class of 3D GeoVolumes

• OGC 3D Tiles and Khronos® glTF™ as payload for 3D content

• Ericsson Texture Compression 2 (ETC2), Khronos® KTX texture formats, PNG and JPEG to encode textures

• OGC GeoPose as a convenient way to encode both position and orientation information

• ISO 19111 (OGC Abstract Topic 2) as the foundation for referencing by coordinates

As part of reviewing the GeoPose standard, issues were documented in the GeoPose GitHub repository. The relevant capabilities that would greatly improve the usability of the standard only exist in advanced targets of the standard which were found to be cumbersome and not a natural extension of the core capabilities of the simple target or of the JSON encoding itself. These capabilities include associating time stamps and identifiers with an individual pose, defining sequences of multiple GeoPoses, and defining a pose relative to a parent pose. A recommendation was made to consider defining these capabilities as extensions preserving the basic form of the simple target. Two space scenarios illustrate what this could look like in a JSON encoding. In one scenario, passengers are onboard a spaceship that is in free flight from Earth to Mars, and in the other, passengers are onboard an express train on the surface of Mars.

As part of reviewing the OGC API — Moving Features standard, issues were documented in the GitHub repository, including clarifying the possible synergy with the OGC GeoPose and OGC API — Connected Systems, and use cases for high performance queries.

The research also reviewed existing coordinate systems for referencing objects in Space and experimented with conversions between:

• the equatorial Barymetic Celestial Reference System (BCRS) and Geocentric Celestial Reference System (GCRS);

• ecliptic coordinate systems (heliocentric and geocentric);

• the International Terrestrial Reference System (ITRS) which forms the basis of the EPSG:4978 (Cartesian) and EPSG:4979 (ellipsoidal) 3D coordinate systems; and

• GeoPose (WGS84 / Local Tangent Plane — East/North/Up).

For several of these conversions, as well as for calculating the positions of the Sun, Earth, Moon, and planets in our Solar System at a given time, the NOVAS C library was used. A table summarizing the functions of the library used in this experiment is presented. The SOFA library is also mentioned as a potential alternative. The BCRS is described as the global standard reference system for objects located outside the gravitational vicinity of Earth on Wikipedia.

glTF™ models provided by NASA as well as the Gaia Sky in Colour from ESA were used to provide a situational awareness in space reflecting the camera position and orientation in a visual demonstration. A scenario tracking the position of the International Space Station (ISS) from a sequence of positions returned by a Web API was also demonstrated, using a detailed glTF™ model of the space station provided by NASA. Another scenario which displays and tracks the position of the Voyager 1 space probe along its journey across our Solar System since 1977 illustrates a use case extending deeper into space, again using a NASA glTF™ model as well as historical data.

Finally, the applicability of the theory of special and general relativity to potential scenarios is explored, also noting cases where it may not be applicable. The concept of time dilation is introduced and illustrated with the thought experiment of a light clock, from which the Lorentz factor is derived simply with the well-understood Pythagoras theorem. The scenario of Global Positioning Systems (GPS) is discussed, where the velocity and gravitational fields have opposite effects on time dilation for the on-board clocks compared to clocks on Earth. Schwarzschild’s exact solution to Einstein’s field equations is also presented, describing with a simple equation time dilation resulting from both velocity and gravitational fields.

3.  Streaming static data referenced to spherical body

The majority of data collected concerns observations and measurements on or near the surface of a spherical body, in particular planet Earth. A large portion of that data is also static. This static data may provide useful situational context for visualizing both other static content, as well as tracking dynamic objects (e.g., visualizing the position of a satellite in orbit above satellite imagery of the Earth). This section explores data access specifications supporting the interoperable selection and transmission of such static information referenced to a particular spherical body, including Earth, but also other spherical bodies such as the Moon or Mars.